Robots are increasingly being employed in tasks that are otherwise dangerous or tedious to humans. The utility of a robot can be increased when tactile sensors are incorporated into the hands or grippers of the robot to enable the robot to "feel" objects in its environment. Ideally, a robotic hand should be able to hold an object tight enough to keep the object from slipping from its grasp, yet be gentle enough not to crush or break the object. One method of accomplishing these objectives is by an elastomeric, three-dimensional sensor tip such as is disclosed in U.S. patent application Ser. No. 07/198,193 filed May 24, 1988, U.S. Pat. No. 4,982,611, and which is incorporated herein by reference. The sensor tip includes transducers that are positioned in selected positions and orientations about the surface of the sensor tip and are preferably composed of a piezoelectric polymer film having lead wires attached. The wires carry the electrical signals that correspond to the deformation or strain experienced by the elastomeric body. The signals are amplified by a signal amplifier, and then are provided to a hard wired processing circuit or a computer which decouples the electrical signals into the individual force components.
In a simple measurement scheme, the electrical signal from the piezoelectric polymer film of a particular transducer created by a step in force may be measured directly by an oscilloscope with a typical 10 megohm (10.times.10.sup.6 .OMEGA.) input impedance. The capacitance of the piezoelectric polymer film is typically approximately 200 pf (200.times.10.sup.-12 f). The voltage appearing across the piezoelectric polymer film will then be drained off through the oscilloscope as a first order decay according to the time constant, .tau., where .tau. equals the resistance (R) multiplied by the capacitance (C). In such a case .tau.=(10.times.10.sup.6).times.(200.times.10.sup.-12)=0.002 seconds. Force information lasting 0.002 seconds is of little use in a system which is intended to grasp and hold objects to perform useful work. Consider, for example, the grasp by a robot of a bolt that is to be inserted into a threaded hole. The grasp may be required for as long as 10-15 seconds. If the force information is to have less than 5 % drift over that period, then, assuming a first order decay, the time constant of the system must be 200 seconds. To provide usable information, therefore, a means for providing quasi-steady state force information without substantial signal drift and without compromising measurement sensitivity is necessary.
The decoupling of the individual force components acting upon the sensor tip is readily accomplished in that the ouputs of the transducers may be related to the forces by various linear and non-linear decoupling algorithms. To provide for proper decoupling, it is necessary to calibrate the sensor tip using known force levels to calculate the constants used in the decoupling algorithm. A force delivery system adequate for calibration must therefore be able to deliver pure forces while simultaneously recording the sensor output. Force delivery systems involving weights and pulleys do not allow good accuracy or repeatability and the use of pulleys introduces high levels of friction. Such force delivery systems are clearly inadequate for testing and calibration of sensor tips with industrial application.
In the repeated use of a sensor tip as described above, it has been found that the piezoelectric polymer film may delaminate from the surface of the outer boundary of the sensor tip. Though the sensor tips are inexpensive and readily consumable, the performance and lifetime of the sensor tip can be improved if such delamination can be prevented.